Drainage System And Modular Drainage Element

ABSTRACT

This invention relates to a drainage system for use in the ground ( 3 ) below ground level ( 4 ). The system comprises at least one module drainage element ( 2 ) provided with a storage space ( 5 ). The storage space is at least partly surrounded by a filter fabric ( 6 ) provided with at least two zones ( 12, 15 ) which are provided with mutually different liquid-permeability.

The invention relates to a drainage system for use in the ground below ground level, the system comprises at least one modular drainage element provided with a storage space, wherein the storage space at least is partly surrounded by a filter fabric.

With the term drainage systems, all systems are meant and included, which are suitable for use in water management including water retention systems. In urban areas, large parts of the ground soil are hard surfaced. Rainfall on paved areas almost directly leads to running off of the water through the sewer system into the urban wastewater treatment. Sewer systems are designed to be able to handle peak flows, and to prevent flooding so that large sewage channels are applied. As a result, the installed maximum capacity of the sewage systems is rarely used, so that these systems are relatively expensive compared to the normal average water load. In order to increase the water retention at peak loads, it is known to provide subterranean basements in the sewage system, so that enlargement or expansion of the sewer system can be prevented or delayed.

The drainage system according to the invention is arranged for use in collecting, retaining (holding) and (re)distributing (local) rain water or other sources of water (irrigation) in order to be able to avoid flooding and to prevent discharging water (too) rapidly into the sewer, and in order to store the water (water retention) and to controllably release and supply the water to dry soil and/or in dry periods. These objectives are achieved by means of elements, which in a certain area, such as a garden of a house or a car parking, are placed into the ground (underground) and are connected to the rainwater (stormwater) runoff system or to other sources of (excess) water. Preferably, the elements are introduced into an excavated (elongated) ditch, which are refilled around the elements with the excavated soil or a different (type of) soil or sediment, such as sand, gravel, loam or clay. Dependent upon the problems resulting from the soil characteristics and its local properties, including the structure and the texture, a solution can be configured comprising a system of drainage elements, complemented with suitable soil around the drainage elements. For example, for dry sandy soil, it is advantageous to improve its retention properties by providing a drainage element that retains water, optionally in combination with improvement of the water retention of the surrounding soil, e.g. by using another type of ground soil around the element, such as clay soil. In clay soil, from which water drains poorly and which has a low infiltration capacity and a low infiltration speed, it is advantageous to apply large drainage elements in combination with a well-draining soil, such as sand.

The infiltration rate is the rate at which water infiltrates into the ground, and is usually expressed in mm of water (column) per day or per hour that is absorbed into the ground. Clay (1-5 mm/h) is one of the types of soil having a low infiltration rate (<15 mm/h). An average infiltration rate has a range of 15-50 mm/h; clay (10-20) and fine sand (<30 mm/h) also fall under this category. Coarse sand (>30 mm/h) falls under the category of high infiltration rate (>30 mm/h).

The infiltration rate is also determined by the water content or the moisture content of the soil. The water content of the soil is usually expressed in millimetres of water (column) present in a meter (dry) soil. For sand the values for the water content are between 25 and 100 mm/m, for loam between 100 and 175 mm/m, and for clay between 175 and 250 mm/m. When a soil is fully saturated with water, and the pores in the soil are completely filled with water so that there is no air present in the soil, the soil is regarded as saturated. Saturated coarse sand will dehydrate completely within a few hours, however for clay soil this will take 2 to 3 days. Both for wet saturated, poorly infiltrating ground, as well as for dry ground having a high infiltration rate, it is advantageous to provide underground a drainage system according to the invention. Two or more modules can be mutually coupled, for example by means of a conduit. If desired, the conduit may also extend through an element and may be provided with drainage openings, so that by coupling the ends of the conduit sections, which ends are located outside the elements, a fluid communication between the conduits and the (rain) water pipe is created, so that a system of coupled modular drainage elements is formed.

From European Patent Application EP1818463 a system is known, comprising a water drainage tank or channel module being provided with gratings or panels, which form side walls and top and bottom walls, in order to define a storage space or storage body. For example, the panels and gratings may be produced by means of extrusion or injection moulding of a plastic material, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) or a combination of (synthetic) materials. The panels are preferably open with an open space of20-80%. The module is wrapped up in a suitable geotextile material, which is at least partially provided around the outer periphery of the storage space—which is defined by the side walls in combination with the top and bottom wall—in order to regulate the outflow and discharge of (rain) water from the module, and in order to prevent inflow of soil sediment into the module from outside, which will pollute the module. In order to form a storage tank or holder from a module, a water impermeable membrane is arranged around at least a part of the outer circumference of the module.

Preferably the geotextile material is manufactured from polyester yarn; yarn from nylon, fibreglass, or polypropylene; which yarns may be woven, knitted, or be of a non-woven type.

From GB2440449 a drainage element is known for use underground. The drainage element is provided with a storage space, wherein the storage space is entirely surrounded by a filter fabric. The filter fabric is manufactured from a geotextile material and is provided with flaps and hook-and-loop straps to be able to close the fabric after folding it around the storage space. The drainage elements according to GB2440449 have the disadvantage that they cannot be easily adapted to the (water) conditions at the site of application.

Furthermore, the above mentioned known systems have the disadvantage that the release of water from the empty space is difficult to regulate, so that only with the aid of a geotextile the release rate can be controlled. When the internal space of the module fills with water, the water flow through the geotextile will increase due to the increased static pressure.

Applying a water impermeable membrane, next or (partly) over the geotextile's material, is costly with respect to the technical production and maintenance, and can lead to (too high) preferential flow in the surrounding soil causing danger of ground displacement or landslip.

The object of the invention is therefore to provide a drainage system, in particular a modular drainage elements, by which water easily and at low cost can be absorbed, retained and be released, wherein said elements are able to be adapted easily and at low cost to the desired improvement of the water management of the ground and the terrain, or to the reduction of the load of the sewage system.

For this purpose the invention provides a drainage system, wherein the filter fabric is provided with at least two zones, which zones are provided with a mutually different liquid-permeability.

Providing the filter fabric (cloth) of the envelope (sleeve) from a suitable geotextile having zones of different liquid-permeability, enables providing a drainage module, which is easily adaptable to the circumstances at the site of application, without requiring costly measures, like applying two or more different filter fabrics.

The filter fabric is preferably provided with zones—arranged in a pattern—having mutually different liquid-permeability. These patterns may include large or small area's, which comprise and cover a large portion of the surface of the filter fabric, but may also be carried out as hatches, such as vertical or horizontal hatching formed by arranging stripes or strips in the filter fabric having a different liquid-permeability.

Preferably, at least one zone in the filter fabric is provided with a gradually changing liquid-permeability, preferably with a gradient. For example, it is advantageous to provide the liquid-permeability within a zone with a linear gradient, ranging from 100% liquid-permeability to 0%. As a result, the liquid intake and liquid discharge, to and from the surrounding soil, can be better and more precisely controlled and can be better adapted to the local conditions. The applied gradient can be chosen as linear, that is to say that the increase of the liquid-permeability per unit length of the zone is constant. If desired, also a non-linear gradient or multiple (non) linear gradients may be provided in a zone.

Preferably, the filter fabric is provided with a zone having a lower liquid-permeability by means of the application of a liquid-repellent coating. Through the use of a coating, or a liquid-repellent substance or composition penetrated into the filter fabric or adhering onto the surface of the filter fabric, in a simple manner the required zones (if applicable: also in the desired pattern) may be arranged into the filter fabric.

A modular drainage element of the drainage system according to the invention may be provided with a filter fabric, in which one of the zones is liquid-impermeable. This causes the drain element, at least partly, to function as a liquid-retention buffer, so that liquid can be retained and stored, and optionally can be controllably discharged.

In a preferred embodiment, the filter fabric entirely surrounds the storage space. In particular, the filter fabric is folded from a filter fabric provided with a bottom side, two side walls, two end faces, and a cover side.

By this alternative arrangement of the (rectangular) drainage element, the liquid from the surrounding soil can be filtered and collected in the storage space. Thereby is prevented that the drainage element, in particular the storage space, pollutes and silts up by soil particles or sediment carried by the liquid. Furthermore the ease of handling of the drainage element improves, when a material is applied in the storage space, which is uncomfortable to handle, such as for instance mineral wool (stone wool). By completely enveloping the mineral wool into a filter fabric, the human skin and other sensitive materials can no longer come into contact with—and become irritated by—the mineral wool fibres.

The bottom side and the two end faces of the filter fabric preferably form a zone with a liquid-permeability which is between 0% and 20%; and the side walls preferably form zones which are provided with a gradually changing liquid-permeability having a lower limit which is between 0% and 20% liquid-permeability and with an upper limit which is between 25% and 100% liquid-permeability; and the cover side preferably forms a zone with a liquid-permeability which is between 80 and 100%. With this configuration of the drainage element and the configuration of the zones upon the filter fabric, the drainage system can be adapted, in a simple and inexpensive manner, to the local conditions, such as soil type, infiltration capacity, average and peak rainfall, including the mode of application such as water buffer or as moisturizing element.

A preferred embodiment for use in clay soil is characterized by a zone with a liquid-permeability of 0% (liquid-impermeable), formed by the bottom side and the two end faces of the filter fabric; and by zones on the side walls which are provided with an gradually changing liquid-permeability of 0% liquid-permeability adjacent the bottom side, up to 100% liquid-permeability adjacent the cover side; and by a zone with a liquid-permeability of 100% on the cover side. This allows (rain) water to flow via the cover side into the drainage element, in order to retain and buffer the water for the most part at the bottom side by the liquid-impermeable (-impervious) zones.

A preferred embodiment for use in sandy soil is characterized by a zone with a liquid-permeability of 0% (liquid-impermeable), formed by the bottom side and the two end faces of the filter fabric; and by zones on the side walls which are provided with an gradually changing liquid-permeability of 0% liquid-permeability adjacent the bottom side, to 25% liquid-permeability adjacent the cover side; and by a zone with a liquid-permeability of 100% on the cover side. As a result, this allows for retaining and buffering the (rain) water flowed via the cover side into the inside of the drainage element and subsequently allows for slowly and controllably discharging this water to the sandy soil, due to the limited liquid-permeability of the zones on the side walls and by the non-limited liquid-permeability of the cover side. This will prevent the rainwater from infiltrating into the groundwater too quickly and will facilitate that the topsoil near the ground level remains moist long after the rain.

These drainage elements are preferably provided with a (rectangular) trunk shape with a cover side having a large surface area for collecting the surface (rain) water as much as possible. The drainage element is preferably provided with a ratio of the length to the width of the cover side, which is in the range of 1:1 to 1:4, at a ratio of the height to the width, which is in the range of 10:1 to 1:1. In a preferred embodiment, the cover side has a length of about 1200 mm and a width of about 1000 mm over the end face, and a height of about 300 mm. Preferably, in alternative embodiments, the dimensions (height×width×length) are selected from: 300×500×1200; 300×300×1200; 150×1000×1200; 150×500×1200; 150×300×1200.

An alternative embodiment is characterized in that the filter fabric has a bag shape, obtained by folding the filter fabric over a fold line and by joining together the side edges in pairs. In particular, the bag shape is turned inside out, so that tip portions, folded onto each other, are extending inside the bag shape.

This embodiment has the advantage that this bag shape is easy to manufacture at low cost, and that the storage space can be easily inserted inside the bag shape. By folding the tip portions inside the bag shape a tight, smooth filter fabric envelope for the drainage element is obtained.

Preferably, a non-woven geotextile is selected for the filter fabric having a density which preferably has a value between 100 and 150 grams per m² and more in particular is approximately 130 grams per m². It has been found that such non-woven geotextile has the best filtering effect for the drainage elements according to the invention.

The edges of the filter fabric are preferably provided with hook-and-loop or (double-sided) tape, by which the cover side can be easily attached onto the edges of the two end faces and the opposite side wall—or with which the lower edges of the side walls of the bag-shaped filter fabric may be interconnected—whereby an entirely enveloped drainage element is formed. This has the advantage over fixed-stitching of the cover side on the side walls of the filter fabric, that no seams are present in the filter fabric through which easily leakage may occur.

Preferably, the storage space of a drainage element is at least partly filled with a porous, liquid-absorbing material. Preferably, the basis of the liquid-absorbing material is selected from: foamed formaldehyde resin; mineral wool including modified mineral wool and mineral wool with a hydrophilic binder; natural and synthetic sponge material; woven and non-woven (geo) textile material. These materials are easy to process, have good properties for storing and dispensing of water at a low price.

By providing the storage space with, for example, a hydrophilic material, the retention and release of water into and out of the drainage element can be better adapted to the requirements defined by the location of application of the drainage system. By applying hydrophilic material, water can be retained better and be discharged more gradually, thereby providing a contribution to the desired improvement of the water management of the soil and the terrain, or to the reduction of the load of the sewage system.

In an advantageous embodiment, a drainage conduit, such as a pipe, is provided in the storage space of the drainage elements, through which conduit liquid can be transported in and out of the modular drainage element. Preferably, this drainage conduit is perforated in the interior of the storage space, so that liquid is able to flow out of the drainage conduit into the drainage element, or in the case of wet, over-saturated soil, liquid is able to flow from the storage space of the drainage element into the drainage conduit in order to be discharged, for example, to the sewer.

The invention also relates to a drainage element for use in the drainage system according to the invention.

The invention will be hereinafter further explained in more detail by means of the drawing of an embodiment of the drainage system containing drainage elements, whereby features and other advantages will come forward.

FIG. 1A shows an embodiment of the drainage system according to the invention, with separate modular drainage elements, surrounded by filter fabric;

FIG. 1B shows an alternative embodiment of the drainage system, wherein the drainage elements are provided with a perforated pipe;

FIG. 1C shows the combined use in the drainage system of rainwater collection and drainage buffer;

FIG. 2 shows in greater detail a filter fabric for a drainage element according to the invention;

FIG. 3 shows the folded-open filter fabric of FIG. 2 according to the invention, provided with zones having different liquid-permeability;

FIG. 4 shows an alternative embodiment of a drainage element, provided with folding zones and a drainage conduit;

FIG. 5 shows the folded-open filter fabric of FIG. 4, provided with folding zones;

FIG. 6A shows a folded-open filter fabric to be folded and interconnected into a bag shape;

FIG. 6B shows a bottom view of the filter fabric of FIG. 6A disposed around a drainage element;

FIG. 6C shows a top view of the filter fabric of FIG. 6B.

FIGS. 1A-1C all show a drainage system 1 for collecting, buffering and redistributing of liquid. In particular, the system is suitable and arranged for absorbing, storing and controlled releasing of water, such as rainwater or surface water. The drainage system comprises at least one drainage module, in other words a modular drainage element 2, which is fitted into the ground 3, below ground level 4. The groundwater 10 or another water-bearing layer is located below the drainage elements. In general, a number of drainage elements are applied in the drainage system, which elements preferably extend in line with each other in a consecutive row .

Each drainage element is provided with a storage space 5 for liquid. This storage space may be hollow and empty; alternatively a material having special properties may be applied, such as a water-absorbing or hydrophilic material. Preferably, at least one drainage element is provided with a filter fabric, which at least partly surrounds the storage space 5 of the drainage element 2 and which extends over the outer surface of the drainage element. The filter fabric is provided with at least two zones, which are provided with a mutually different liquid-permeability.

In FIG. 1A an embodiment is shown for receiving, storing and releasing of a surface rainwater 7. The rain(precipitation)water falls on the ground surface 4, and penetrates into the ground soil 3, and then flows into the drainage elements 2 through the upper side, that is the cover side of the drainage elements 2, which are arranged below ground level (underground). Depending on the type of soil and the average moisture content of the ground, the drainage elements are provided with means for regulating the inflowing rainwater, such as zones with different liquid-permeability. When the ground is generally dry, or dries out quickly, such as sandy soil or sand, preferably measures are taken in order to retain the received water longer and to regulate the slow release of the water to the surrounding soil.

In FIG. 1B, an alternative embodiment is shown of the drainage system 1 according to the invention. The drainage elements 2 are provided with a drainage conduit (pipe) 8, which extends from one end face to the other end face. The drainage pipes of the drainage elements are suitable for coupling together to create a fluid connection therebetween. Preferably, the drainage pipe is provided with perforations or openings, so that liquid is able to flow in and out of the storage space. For example, the drainage pipe of FIG. 1B is connected to a precipitation runoff system 11, from which (rain) water from a gutter or from a large paved ground surface, such as a parking space, flows into the drainage elements in order to be absorbed and thereafter to be controllably released. In FIG. 1B, the level of the groundwater is high, so that the (ground) water is able to flow through the drainage elements into the drainage pipe. The drainage pipe 8 can be connected to the sewer 9 so that the drainage system here facilitates in draining off the ground 3 around the drainage system 1.

In FIG. 1C, a configuration of the drainage system according to the invention is shown, provided with combined functions and drainage elements. The left two drainage elements 2 are provided with a drainage pipe 8, which is connected to the rainwater runoff system 11. In addition to the two drainage elements having a drainage pipe, another drainage element is arranged, which is not provided with a drainage pipe. This element is fed with rainwater 7 at its upper side. The element on the right hand side in the figure is also provided with a drainage pipe, which is connected to the sewer, so that at this position in the drainage system (welling) groundwater 10 can be discharged to the sewer, e.g. when the water content in the soil is high.

In FIG. 2, a filter fabric 6 according to the invention is shown, for example manufactured from a non-woven geotextile, for use as release-regulating means around a storage space of a drainage element. In this embodiment, the drainage element has a substantially rectangular shape, and preferably is disposed lying flat on its bottom side, as a suitcase model having a cover with a large top surface. The filter fabric 6 comprises a bottom side 12, two side walls 13 and two end faces 14, and a cover side 15. Between the side walls and the end faces 14 folding zones 16 are provided, which in use are being folded against an adjacent side wall 13 or end face 14. Furthermore, the filter fabric may be provided with a hook strip 17 or double-sided adhesive tape for easily closing the cover side 15 upon the end face 14 and/or the opposite side wall 13. Preferably, the hook strip is attached at the underside of the edges of the cover side 15 by means of stitching or sewing.

Preferably, the upper edges of the end faces 14 and the side wall 13, which edges engage the hook strip upon the cover side 15, are provided with a loop strip; preferably these strips are also attached by means of stitching. If desired, the hook strip and the loop strip of the hook-and-loop fastener may change position, and/or projecting parts or flaps may be arranged for fixing the hook-and-loop strips on the end faces, side walls and/or the cover side.

This ensures that the four sides of the drainage element all have the same liquid-permeability, and that leakage is prevented through the stitching seams, in case the cover side 15 by means of stitching is connected to the end faces 14 and the side wall 13.

In FIG. 3, the folded-open filter fabric 6 of FIG. 2 is shown with a bottom side 12, two side walls 13 and two end faces 14, and a cover side 15. In the folding zones 16 preferred fold lines 19 are indicated by dotted lines. The filter fabric 6 in FIG. 3 has been treated, so that zones with mutually different liquid-permeability are present. The rate of the liquid-permeability is indicated in FIG. 3 by means of hatching lines: the thicker the line, the lower the liquid-permeability. In this embodiment the filter fabric 6 is provided with a zone having a low liquid-permeability; this zone is formed by the bottom side 12 and the end faces 14. The cover side 15 is not provided with a restriction of the liquid-permeability so that its liquid-permeability is equal to the liquid-permeability of the untreated filter fabric material. This is indicated by 100% (relative) liquid-permeability. The side walls 13 are provided with zones having a gradually changing liquid-permeability, for example, of a value between 0 and 20% near the bottom side 12 to a value between 80 and 100% near the upper edge.

In a special embodiment, the zone formed by the bottom side 12 and the end faces 14 is provided with a liquid-permeability of 0%, and is therefore liquid-impermeable. This can simply be achieved by moving the filter fabric entirely in the vertical direction in FIG. 3 and applying a (narrow, thin) liquid-repellent layer across the entire width of the filter fabric, in the horizontal direction of FIG. 3, consisting of the adjacently connected end face 14, bottom side 12 and end face 14. For example, the side walls 13 are provided with a zone having a vertical (linear) gradient in the liquid-permeability ranging from 0% near the bottom side 12, up to 25% close to the cover side 15. Preferably, this layer is provided by means of thin stripes/strips liquid-repellent over the entire width of the filter fabric, consisting of two folding zones 16 with the interposed side wall 13. The vertical movement during the application of the liquid-repellent strips easily provides zones having a mutually different liquid-permeability in a vertical direction, wherein the zones have equal liquid-permeability over the width of the filter fabric.

Preferably, the above-described type is used in a drainage system in dry ground according to FIG. 1A, in which rainwater 7 via the 100% (relative) liquid-permeability of the untreated cover side 15 can be easily collected and stored in the storage space of the drainage element. The retained water within the drainage element is slowly released into the surrounding ground soil, so that the infiltration rate is decreased. Because of the liquid-impermeable bottom side 12 and end faces 14, and because of the maximum liquid-permeability of 25% in the side walls 13, the water can not rapidly flow downward, and not flow sideward through the end faces of the drainage system, into the groundwater 10, which is parallel to the row of drainage elements (see FIGS. 1A-1C). Through the side walls 13 the buffered liquid is able to flow from the storage space to the surrounding ground 3, preferably in the upward direction, since the liquid-permeability within the zone of the side wall increases in the direction of the ground surface 4. As a result, the top layer of the ground soil 3 will remain moist, which is advantageous e.g. for shallow rooted plants in dry soil. In FIG. 3, the liquid-permeability is schematically indicated by means of the thickness of the hatching (lines) used. The depicted hatching can also be taken literally as a pattern of stripes of, for example, a liquid-repellent coating, wherein the liquid-permeability is proportional to the depicted width of the white stripes of untreated filter fabric.

In an alternative embodiment for use in clay soil, the liquid-permeability of the zones on the side walls 13 gradually changes from 0% near the bottom side up to 100% close to the cover side 15. As a result, the water is more easily released or retained into the upper ground layers 3 near the ground level 4.

FIG. 4 shows in more detail a modular drainage element 2 of the drainage system 1 of FIG. 1B. Because the element possesses a greater height than width, this element is in particular suitable for the drainage into the groundwater of rainwater originating from gutters or liquid-impermeable paved ground surfaces, and for increasing the infiltration rate. The drainage element 2 in FIG. 4 is provided with a drainage pipe 8, which extends from the one end face to the other end face 14. The drainage pipe 8 is provided with perforations or openings 18 through which liquid, such as (rain) water, is received, absorbed and stored in the storage space 5. A filter fabric surrounds the storage space 5, thereby preventing entry of soil particles and contamination of the storage space. The folding zones 16 are indicated by the dotted, diagonally extending fold lines 19, the folding zones being partly folded-over each other behind the side wall 13.

In FIG. 5, an unfolded filter fabric is shown for application around the drainage element 2 of FIG. 4. On the end face 14, the flange 20 of the drainage pipe is shown. In this embodiment, the filter fabric is not treated with a coating, so that no zones with different liquid-permeability are formed via this method. Provided on top of each other two or more layers of the filter fabric form the zones with a lower liquid-permeability in FIG. 4. In FIGS. 4 and 5, the folding zones 16 are folded against the side wall 13 so that there are zones formed of one 13, three 16 and five 21 layers of filter fabric. It is also possible to provide zones with different liquid-permeability in an alternative way, such as for example by cutting pieces of filter fabric, and applying these pieces on the filter fabric of the drainage element.

The drainage element 2 of FIG. 4, having e.g. folding zones 16 that are folded at the upper side of the side walls 13, is suitable for providing a rapid infiltration of rainwater into the groundwater, as is shown by the two drainage elements 2 at the left-hand side in FIG. 1C. When this drainage element 2 is placed into the ground with its folding sides 16 directing downwards, to the contrary a reduction of the infiltration rate is obtained.

In FIGS. 6A-6C an advantageous alternatively folded filter fabric 30 is shown for use in a drainage element. End face portions 33 and 34, which are connected to each other with their side edges 37 a and 37 b, form the end faces of the element. For this purpose, the tip portions 40, 41, 35 from the upper side 31 on both sides are folded onto each other and inserted inwards, as is shown in FIG. 6C by means of the dotted lines. The filter fabric 30 is closed at the bottom side—for example with a suitable means or method, including two-sided adhesive tape, hook-and-loop fastener, and sewing—by folding the fold flaps 43 and 44 over the bottom of the storage space 5 of the drainage element, and then adhering the lower edges 38 & 39 of the side walls 32 & 36 onto each other.

The filter fabric 30 of FIGS. 6A-6C may advantageously be pre-formed as a bag, by folding the filter fabric about fold line 47 and next imposing one half upon the other half of the fabric, by laying side wall 36 onto side wall 32, so that the area's of FIG. 6A are positioned onto each other inside the bag to be formed. Thereafter for example by applying a stitched seam, the side edges are joined together in pairs 37 a & 37 b and 37 c & 37 d to form a bag having an open side at the lower edges 38 & 39. Subsequently, the formed bag is turned inside out/reversed so that the area's of FIG. 6A reappear at the outside of the drainage element. The thus formed bag can easily be slid onto a storage space 5 and then be closed at the bottom side.

Moreover, the folding method for folding the filter fabric into a bag, as described here above, has the advantage that easily a drainage element may be formed having a low liquid-permeability at the bottom side and a high liquid-permeability at the upper side. By providing a zone with a decreasing gradient in liquid-permeability across the entire width of the filter fabric of FIG. 6A, from the side edges 38, 43 up to the fold line 45 and from the side edges 39, 44 up to the fold line 46, an envelope of filter fabric is obtained, having an identical gradient in liquid-permeability in the vertical direction all-around on the side walls (surfaces 32 & 36) and on the end faces (surfaces 33 & 34).

In particular the storage space of a drainage element is filled with a porous, liquid-absorbing material, so that the retention and release properties of the element can be adapted further to the local requirements and needs for collecting and releasing water. Preferably, a water-absorbing or hydrophilic material is used, including foamed formaldehyde resin, mineral wool including modified mineral wool and mineral wool with a hydrophilic binder, natural and synthetic sponge materials, woven and non-woven textile material. The filter fabric is preferably made of a non-woven (geotextile) material, due to its soil-resistant properties and excellent hydrodynamic properties. 

1. Drainage system, for use in the ground (3) below ground level (4), the system comprises: at least one modular drainage element (2) provided with a storage space (5), the storage space is at least partly surrounded by a filter fabric (6) provided with at least two zones (12, 15), which zones are provided with a mutually different liquid-permeability.
 2. Drainage system according to claim 1, wherein the at least two zones of the filter fabric (6) having mutually different liquid-permeability are provide in a pattern.
 3. Drainage system according to claim 1, wherein the filter fabric (6) is provided with at least one zone having a gradually changing liquid-permeability.
 4. Drainage system according to claim 1, wherein the filter fabric (6) is provided with a zone having a lower liquid-permeability by means of application of a liquid-repellent coating.
 5. Drainage system according to claim 1, wherein one of the at least two zones in the filter fabric (6) is liquid-impermeable.
 6. Drainage system according to claim 1, wherein the filter fabric (6) entirely surrounds the storage space (5).
 7. Drainage system according to claim 6, wherein the filter fabric (6) is folded from a filter fabric (6) provided with a bottom side (12), two side walls (13), two end faces (14), and a cover side (15).
 8. Drainage system according to claim 7, wherein the filter fabric (6), the bottom side (12) and the two end faces (14) form a zone having a liquid-permeability which is between 0% and 20%, and wherein the side walls (13) form zones which are provided with a gradually changing liquid-permeability having a lower limit which is between 0% and 20% liquid-permeability and having an upper limit which is between 25% and 100% liquid-permeability, and wherein the cover side (15) forms a zone with a liquid-permeability which is between 80 and 100%.
 9. Drainage system according to claim 6,wherein the filter fabric (6) has a bag shape, obtained by folding the filter fabric over a fold line (47) and by joining together the side edges in pairs (37 a & 37 b) and (37 c & 37 d).
 10. Drainage system according to claim 9, wherein the bag shape is turned inside out, so that tip portions (40, 41, 35), folded onto each other, are extending inside the bag shape.
 11. Drainage system according to claim 1, wherein the filter fabric (6) is a non-woven.
 12. Drainage system according claim 1, wherein the storage space (5) of a at least one modular drainage element (2) at least partly is filled with a porous, liquid-absorbing material.
 13. Drainage system according to claim 12, wherein basis of the liquid-absorbing material is selected from the group consisting of: foamed formaldhyde resin, mineral wool, modified mineral wool with a hydrophilic binder, natural and synthetic sponge material, and woven and non-woven textile material.
 14. Drainage system according to claim 1, wherein a drainage conduit (8) is provided in the storage space (5) of the at least one modular drainage elements (2), through which conduit liquid can be transported in and out of the at least one modular drainage element.
 15. The drainage element for use in the drainage system (1) according to claim
 1. 